With complete authority (7)

I believe the answer is:

warrant

'authority' is the definition.
(I've seen this before)

'with complete' is the wordplay.
'with' becomes 'w' (abbreviation).
'complete' becomes 'arrant' (I've seen this before).
'w'+'arrant'='WARRANT'

(Other definitions for warrant that I've seen before include "Justify; authorisation" , "Permit for arrest, search of premises, etc" , "Authorise, justify" , "Guarantee" , "Document giving authority" .)

I've seen this clue in The Telegraph.

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